It has been known that diorganopolysiloxanes can be polymerized in the presence of a quaternary phosphonium hydroxide catalyst to increase the molecular weight of the siloxanes. U.S. Pat. No. 4,008,261 to Brown et al., for example, describes a method for increasing the molecular weight of diorganopolysiloxanes by mixing a catalytic amount of quaternary phosphonium hydroxide with a relatively low molecular weight diorganopolysiloxane and heating the resultant mixture at a temperature of from 30.degree. C. to about 130.degree. C., while removing all the water that is present until the desired molecular weight is effected. The quaternary phosphonium hydroxide employed as a catalyst in the method described by Brown et al. is deactivated in the presence of moisture.
Therefore, it is an object of the present invention to provide a method for polymerizing diorganopolysiloxanes by heating the siloxanes in the presence of a quaternary phosphonium hydroxide which can be easily dispersed in the diorganopolysiloxane to be polymerized. Another object of the present invention is to provide a method for accelerating the polymerization of diorganopolysiloxanes. Still another object of the present invention is to provide a method for polymerizing diorganopolysiloxanes in the presence of a quaternary phosphonium hydroxide to form high viscosity siloxanes in which the quaternary phosphonium hydroxides are not deactivated by the presence of water formed during condensation before the high viscosity diorganopolysiloxanes are formed. Still another object of the present invention is to provide a method for polymerizing diorganopolysiloxanes in which the diorganopolysiloxanes thus obtained have less than 30 ppm (parts per million) by weight of Si-bonded hydroxyl groups, with the simultaneous use of an organosilicon compound which controls the chain-length. A further object of the present invention is to provide a method for polymerizing diorganopolysiloxanes in which the quaternary phosphonium hydroxide can be deactivated by heating to at least 135.degree. C. after the desired molecular weight is achieved.